If you have binoculars, a telescope or a telescope, you have to attach this Mylar sheet in front of the lens, never on the eyepiece because the light is too concentrated and burn the foil. Attach it as you can in front of the telescope, with sticky paper or an elastic band as shown in the picture on the left. Do not worry if the sheet is a little crumpled or curled, it does not cause any effect on the image.
Once the leaf is in place, you can only aim for the Sun and observe sunspots safely. Take advantage of this to cover the viewfinder with a cap or a piece of Mylar because it is not protected and the reticle placed in the center (the cross) may peel off in the heat.
The second solution is to project the image of the Sun on a cardboard or metal screen placed behind the eyepiece, about 30 or 50 cm apart. This screen must be white to bring out sunspots. In this system there is no filter; you must never look at the sun through the telescope or the telescope. You must try to find the Sun by gently moving the instrument. As soon as the projected image becomes very bright, you are not far from the Sun. Move the instrument slightly further until the disc appears on the screen.
This solution is ideal if you want to show sunspots to your friends. Only disadvantage, some cheap astronomical glasses do not support the intense heat and you risk to take off the lenses of your lens. In this case it is better to observe the Sun in the late afternoon when its light is less intense. With mirror telescopes there is no problem of this type because the light is reflected on the mirror. But be careful with the eyepieces because they will quickly heat up and maybe get damaged if they are of poor quality.
You can also buy a Herschel helioscope. It is a diagonal reference that you place just before the eyepiece. It has a prism or a semi-reflective mirror that returns outward more than 99.9% of the sunlight. Less than 0.1% of the light passes through the eyepiece. You can observe the Sun by simply adding a gray filter to the eyepiece. In this configuration you are not likely to damage your eyes or melt or explode the filter because the light is no longer intense or hot. This accessory is however quite expensive and often used by amateurs.
The last method is the interference filter. It is a technique a little different, very special, which requires a very expensive accessory because its manufacture is quite complex. These sunglasses are offered by several manufacturers including Meade under the brand name Coronado, Daystar and Lunt.
This filter that looks like a round box is fixed either in front of the lens or instead of the eyepiece of the telescope. It isolates a tiny part of the sun’s light, which is called a spectral line located in the red part of the spectrum, the alpha hydrogen line.
In this light called monochromatic (one color) the Sun appears all red and you can observe the bright and active regions (sunspots) on the surface of the Sun and the protuberances, huge ejections of material often larger as the Earth, which gush slowly from its surface. It is a fascinating spectacle that will hold the whole family because you must observe the Sun during the day.